The present invention is directed to an improved technique for tracking the position of a light pen across a display surface such as the face of an ac plasma panel.
In many computer-based display systems, the computer is capable of "tracking" the location of a so-called light pen as it is moved across the display surface by a human operator. "Tracking" in this sense means identifying successive closely-spaced points along the pen's path of motion. A display system with this capability can be configured, for example, to provide the operator with the ability of moving a distinctive symbol "or cursor," across the displayed image--a particularly useful tool in such applications as computer-aided design. In other applications continuous curves may be drawn by connecting successive identified pen positions with straight line segments. As long as the distance between the successively-identified positions is small, a relatively smooth, continuous line is generated in the displayed image under the path followed by the pen. This creates the subjective impression that the line was "written" by the light pen.
The strategy employed to track the position of a light pen across the face of a display medium varies with the medium. In cathode ray tube (CRT) systems, for example, the light pen signals the computer the instant that refresh information applied to the CRT causes the phosphor adjacent to the pen tip to emit light. Data identifying the point on the CRT surface being refreshed at any given time is correlated with the light pen signal to identify the location of the pen on the CRT surface. Identification of the pen location in successive scans of the display surface allows the pen location to be tracked over time, and since the entire display surface is typically scanned at 30 frames/second, even very rapid movements of the pen can be followed. See, for example, U.S. Pat. Nos. 3,653,001 issued Mar. 28, 1972 to W. H. Ninke and 3,389,404 issued June 18, 1969 to R. A. Koster.
Other display media may require a different approach. The ac plasma panel, for example, is a display medium comprised of a matrix of individual display sites each of which resides at any given time in one of two possible states--ON (light-emitting, energized) or OFF (non-light-emitting, non-energized). In the ON state, a site emits a train of individual light pulses at a sufficient repetition rate, e.g., 50 kHz, that the site appears to be continuously light-emitting. In contradistinction to the sequential scan operation of a CRT, however, all ON sites of an ac plasma panel emit their light pulses at the same time. Thus, the normal ON site light pulses cannot be used to determine the particular site over which the pen is located. However, an individual site can be "excited," i.e., made to emit a light pulse, when the ON sites are not emitting light. In order to determine the pen location, then, the sites are excited in a predetermined "scanning" sequence. Once the light pen detects a light pulse at a nonstandard time, its location becomes known since the location of the last site to be excited is known. Such an arrangement is shown, for example, in U.S. Pat. No. 4,099,170 issued July 4, 1978 to S. R. Maliszewski et al.
Disadvantageously, it takes a relatively long time to scan all the sites of a plasma panel to search for the light pen. Thus, a light pen tracking arrangement based on such a scanning strategy (in analogy to the CRT arrangements) may be unable to smoothly track even moderate movements of the pen; successively identified pen locations are spaced too far apart, both temporally and spatially. One technique which goes a long way toward solving this problem is taught in U.S. Pat. No. 3,938,137 issued Feb. 10, 1976 to P. D. Ngo et al. Once an initial location of the pen is determined, e.g., by scanning the entire panel, subsequent scans are confined to a small area, or patch, centered at the last known pen location. The underlying theory of this approach is that the pen will not have moved outside of the scanning patch since the last scan. This technique does, in fact, enable the new pen position to be identified much more quickly than when the entire panel is scanned. However, our experience has been that it is still possible for a user to move the pen with sufficient rapidity to cause the pen to "outrun" the patch if the latter is fairly small. The entire panel must then be scanned again, resulting in slow reacquisition of the pen location. It is, of course, possible to simply use a larger patch. However, because more sites must be scanned, the average time needed to locate the pen is increased, resulting in inaccurate tracking of its path.